1. Field of the Invention
The present invention relates to control means for automatic transmissions and more particularly to control means for automatic transmissions adapted to be used in motor vehicles such as automobiles.
2. Description of Prior Art
Conventional automatic transmissions generally include a hydraulic torque converter having an output shaft connected with a multiple-stage gear transmission mechanism such as a planetary gear mechanism. For selecting a desired one of the gear stages, the transmission usually includes a hydraulic system which has hydraulically operated or electromagnetically operated valves for appropriately selecting hydraulic circuits to thereby engage selected ones of friction devices such as clutches and brakes. Where the hydraulic system includes electromagnetically operated solenoid valves, electronic means are generally provided for detecting that the vehicle operating condition has been shifted from one zone to another across a shifting line and for producing electric signals for energizing an appropriate one or ones of the solenoid valves.
Conventionally, such a shifting line is determined in terms of the vehicle speed and the engine load and sometimes has one or more stepped portions. Since the vehicle speed is dependent on both the engine speed and the selected gear stage, it has been required in such a conventional arrangement to provide shifting lines of different patterns for respective pairs of adjacent gear stages. Further, since the engine load is usually detected stepwisely in terms of the engine throttle valve position, there has been a possibility that the shifting lines do not match the engine operating characteristics, particularly in such areas where the shifting lines are of stepped configuration.
In order to eliminate the above problems, Japanese patent publication No. 56-44312 corresponding to U.S. Pat. No. 4,027,554 proposes to determine the shifting line in terms of the torque converter turbine speed and the engine load. With this proposal, one shifting line can be used throughout the gear stages. Further, the proposal is also considered advantageous in that the torque converter turbine speed is relatively stable even under a wide change in the engine throttle valve opening so that hysteresis between shifting-up and shifting down lines can be made small. Further, it is not necessary to provide any restricting line, such as a stall line, so that there is provided versatility of determining the shifting lines.